Comments 0

Presentation transcript

Comparison of acquisitionTechniques for GNSS SignalProcessing in Geostationary Orbit

B.Chibout, ENAC/TesA

C.Macabiau, ENAC

A-C.Escher,ENAC

L.Ries, CNES

J-L.Issler, CNES

S.Corrazza,ThalesAleniaSpace

2

Introduction

GPS is optimised for an earth use.

The use of GNSS for geostationary positioning ispossible with specific constraints.In particular, thegeostationary satellites have to process signals with lowC/No (<25dBHz) to ensure that at least 4 satellites canbe used to compute its position

Our aim is to design an autonomous geostationaryreceiver

In geostationary orbit, the receiver faces long spellwithout the possibility to demodulate the data for eachsatellite. This fact lays a problem to compute its positionautonomously.

To solve this problem, a reduction of the datademodulation threshold is envisaged to increase thenumber of GPS satellites with valid ephemeris.

3

Introduction

Three differents acquisition techniques are thenpresented.

-1+1ms FFT acquisition method

-

Half Bit acquisition method

-

Double Block Zero Padding Method

Their performances in geostationary orbit with thespecific constraint (number of useable satellites) arecompared over a 24 hours period

A peak acquisition extrapolation technique is alsopresented to improve the accuracy of the positioning

4

Outline

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

REDUCTION OF THE DATA DEMODULATIONTHRESHOLD

ACQUISITION

ALGORITHMS

–1+1ms

FFT

acquisition

method

–Half

Bit

acquisition

method

–Double

Block

Zero

Padding

Method

GEOSTATIONARY

ACQUISITION

RESULTS

5

Outline

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

REDUCTION OF THE DATA DEMODULATIONTHRESHOLD

ACQUISITION ALGORITHMS

–1+1ms FFT acquisition method

–Half Bit acquisition method

–Double Block Zero Padding Method

GEOSTATIONARY ACQUISITION RESULTS

6

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

Geostationary

Orbit

GPS Orbit

Earth

GPS 1

GPS 2

S’

S

45°

S’’’

Teta GEO

S’’

GPS

main lobe

main and secondary lobes

main lobe only

•

For a geostationary receiver, theusefull GPS satellites are mainlythose located on the «

opposite

»sideof the earth

•The GPS satellites located in the S’’area are masked by the earth.Masking Earth encompasses the Iono

•The free space losses are 5 to 10dBhigher

than for an earth user

•High DOP value (from 15 to morethan 50)

•The gain of the transmitting antennadecreases drastically when signalsare emitted through side lobes

•

The design of the receiver must beadapted to that geometry

7

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

Receiver antenna pattern: 9 dBfrom 0°

to 30°

and decreasingvalue after

In red: theoreticall pattern

In blue: receiver pattern wellsuited to GEO condition.

The received C/No decreases with the offboresight angle of the GEO antenna.

C/No are mainly between 18 and 27 dBHz

The acquisition techniques have to deal withthese low values

Main lobe only receiver Antenna pattern

8

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

246810121400.10.20.30.40.50.60.70.80.9PROBABILITY TO SEE AT LEAST n SATELLITES WITH C/No >20NUMBER OF SATELLITESPROBABILITYY123456780.10.20.30.40.50.60.70.80.91PROBABILITY TO SEE AT LEAST n SATELLITES WITH C/No >25NUMBER OF SATELLITESPROBABILITYY

The simulations are conducted with the nominal 24 satellites GPS constellation

The number of useable satellites increases while the processing threshold decreases.

More than 65% of the time, the GEO receiver sees 4 satellites and almost every time 2satellites with C/No>25 dBHz.

For 100% of the time, the GEO receiver sees 6 satellites with C/No>20 dBHz

A 20 dBHz processing threshold should be sufficient to provide continuouspositioning

9

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

0510152025050100150200250300350400•To be autonomous and get precise position, the receiver must be able todemodulate the data of each PRN so as to get their ephemeris.

•

The C/NO of the received signals have long spell under the classic datademodulation threshold: 27 dBHz.

PRN number

Time

(min)

TOTAL VISIBILITY DURATION WITH C/NO>27dBHZ OVER 1DAY

10

TOE1

TOE2

Validity period for ephemerisbroadcasted with TOE1

Validity period for ephemerisbroadcasted with TOE2

t

t-2h

t+2h

t+4h

Ephemerisbroadcasted withTOE1

Ephemerisbroadcastedwith TOE2

Once an ephemeris set is demodulated, itsvalidity is 4h. A new set of ephemeris is broadcastevery 2hours.

As long as the C/NO of a GPS satellite is abovethe Data Demodulation Threshold (i.e 27 dBHz),the ephemeris is known for the next 4hours in thebest case

GPS SIGNAL CHARACTERISTICS FOR AGEOSTATIONARY ORBIT RECEIVER

the ephemeris demodulation cannot always be achieved (inspite of their 4 hours

validity)

the number of useable satellites to compute the GEO satellite position decreases

Three acquisition techniques in geostationary environment have been studied

The number of usable GNSS satellites is increased by lowering the DataDemodulation Threshold

To obtain almost continuous positioning, i.e to successfully acquire at least 4 usablesatellites at any time, the duration of the signals to be processed during the acquisitionmust be very long for the first two methods(1+1ms method and Half Bit method):up to 3seconds. The double block zero padding method needs less signal

The 1st two methods have heavy computational cost and duration due to the durationof the signal processed. The double block 0-padding method is more than 20 timesfaster

The accuracy of the positions are similar with the first two methods. Lower accuracywith the double block zero padding method.

The average position error is large (>750m) because of the large DOP and theresolution of the pseudoranges at the output of the acquisition process (only 1/2 chip)